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A submarine canyon as the cause of a mud volcano — Liuchieuyu Island in Taiwan
Marine Geology 176 (2001) 55±63
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A submarine canyon as the cause of a mud volcano Ð Liuchieuyu Island in Taiwan J. Chow a,*, J.S. Lee a, C.S. Liu b, B.D. Lee c, J.S. Watkins d a
Institute of Applied Geophysics, National Taiwan Ocean University, 2, Peiling Road, Keelung, 20224 Taiwan, ROC b Institute of Oceanography, National Taiwan University, Taiwan, People's Republic of China c Energy and Resources Laboratory, Industrial Technology Research Institute, People's Republic of China d Department of Geology and Geophysics, Texas A & M University, College Station, TX 77843-3115, USA Received 21 July 2000; accepted 21 March 2001
In this paper, based on 3.5 kHz, UNIBOOM and conventional seismic data, we propose a model for the creation of Liuchieuyu Island, a near-shore mud volcano off the southwestern coast of Taiwan. In support of this model, we also discuss the relationship between a nearby submarine canyon (Kaoping Submarine Canyon) and the mud diapirs and mud volcanoes in the region. Seismic data suggest that Liuchieuyu was originally a mud diapir with a thick, continuous accumulation of sediments on its upper surface. This sediment load or overburden prevented uncon®ned growth of the underlying mud diapir, while simultaneously acting as a seal that trapped gas and formed a high-pressure zone. However, Kaoping paleo-canyon eroded these overlying sediments, and the overburdening pressure was reduced. In consequence, the mud diapir rose up to the sea ¯oor to become a mud volcano, the uppermost part of which is present-day Liuchieuyu Island. The formation of this island in the path of the paleo-canyon also diverted the channel of the canyon to its present location. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Taiwan; Seismic re¯ection; Submarine canyon; Mud volcano
In the early Pliocene, the paleoenvironment of the offshore area of southwestern Taiwan was deep marine (Covey, 1984). Large amounts of mud and silt derived from the Penglai orogeny were deposited in this region during that time, and in the Pliocene, a thick ®ne-grain sediment sequence was formed. However, as the arc-continent collision developed, the depositional environment in this area became
* Corresponding author. Tel.: 1886-2-24622192 ext. 6314; fax: 1886-2-24625038. E-mail address: [email protected] (J. Chow).
shallow marine and deltaic, and the compressive tectonic forces of this still-active collision together with the unbalanced sediment load combined to trigger the mud diapirism in this region. Disturbance of the very young sedimentary strata on top of several mud diapirs suggests that they are still active (Chow et al., 1996). Liuchieuyu Island, which is about 12 km off the southwestern coast of Taiwan (Fig. 1), is the only mud volcano in this offshore area to have emerged above sea level, even though there are many mud diapirs nearby. The island is located opposite the mouth of the Kaopinghsi River, and next to the Kaoping Submarine Canyon. Its northeast±southwest axis is about 5 km long, and its maximum width in the
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00157-8
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J. Chow et al. / Marine Geology 176 (2001) 55±63
Fig. 1. Map showing the locations of Liuchieuyu Island, the UNIBOOM seismic pro®les (U1±U4), the 3.5 kHz pro®les (A±J) and the conventional seismic sections (inset). Bathymetric contour is at 50 m interval. FC in the inset is Fangliao Canyon. The thick lines on the UNIBOOM traces and on the conventional seismics in the inset indicate the seismic sections shown in the corresponding ®gures.
northwest±southeast direction is 3 km. The total area above sea level is about 10 square km. The island is composed of greenish-gray mudstone from the late Pliocene, covered with a 4±10 m thickness of reef limestone that was deposited in the Pleistocene (Shih et al., 1991).
2.1. Conventional re¯ection seismic sections Two-channel re¯ection seismic pro®les were acquired aboard the R/V Ocean Researcher I during
J. Chow et al. / Marine Geology 176 (2001) 55±63
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cruise 320 (Fig. 1, inset). The total length of these pro®les was about 521 km. The seismic source used was an array of three Bolt airguns with a maximum output volume of 920 in 3. (500 in 3 plus 300 in 3. plus 120 in 3.) at a pressure of 2000 PSI. Positioning was maintained using a Trimble Nav Graphic XL Global Positioning System and all the seismic data were digitally recorded. The seismic re¯ection data were processed using the SIOSEIS seismic data processing system.
was good enough not to require any subsequent special processing of the digital record.
2.2. High-resolution shallow seismic data
3.1. Liuchieuyu mud volcanoes
Since the high seismic source frequency of UNIBOOM pro®les makes them very suitable for high-resolution study of shallow shelf areas, four NW±SE UNIBOOM pro®les, of total length 34.8 km, were acquired northeast of Liuchieuyu Island (Fig. 1). These pro®les run from the margin of the Kaoping Submarine Canyon in the NW, to the coastal shelf in the SE. The seismograph used was the UNIBOOM 230-1 (EG&G) provided by Energy and Resources Laboratories, ITRI. This UNIBOOM system provides a sonic-impulse seismic source with an energy of up to 300 J. The generation rate is 3.3 impulses per second with a main frequency range of 400 Hz , 14 kHz. In 300 m of water, the stratigraphic resolution is 15 , 30 cm, and penetration depth is 30±60 m. During acquisition, the vessel speed was kept at about four knots and a Trimble Nav Graphic XL Global Positioning System was used to maintain position and record the pro®le location. Data was recorded up to a two-way time of 250 ms. Since the quality of the pro®les was good, no processing was done on the data set except for the bandpass ®ltering (low cut 800 Hz, high cut 4 kHz) used during the acquisition.
To the northeast of Liuchieuyu Island, the conventional seismic section (Fig. 2) and the high-resolution UNIBOOM shallow seismic pro®les (see Fig. 3) show a structureless mud volcano, about one km wide. This submarine mud volcano pierces the sedimentary cover, and the surrounding sedimentary strata dip away from it (Fig. 2 and 3). Fig. 3 also shows a crater that was formed by the natural rapid release of large volumes of over-pressured gas. Structurally, this crater conforms to the three-dimensional model used by Prior et al. (1989) to interpret a similar mud volcano crater in the Gulf of Mexico. These data further con®rm that Liuchieuyu Island itself is the above-sea-level part of a large submarine mud volcano. The extent of the submarine part of this volcano is indicated in Fig. 4.
2.3. 3.5 kHz Echo pro®les During cruise 434 of the R/V Ocean Researcher I, about 130 km of 3.5 kHz echo pro®les were acquired. These pro®les were located around Liuchieuyu Island, with some of them crossing Kaoping Submarine Canyon (Fig. 1). During the acquisition of the 3.5 kHz pro®les, the vessel speed was kept about at 5.5 , 6.5 knots, and the on-board GPS was used to record the pro®le location. The quality of the data
The following interpretations of the seismic pro®les are based on analytical techniques that have been described elsewhere (e.g. Sangree and Widmier, 1979; Bouma et al., 1983, 1989). Fig. 1 shows the locations of all the seismics discussed below.
3.2. Gas seepage The presence of gas is suggested by a small, acoustically re¯ective plume in the water column immediately above the crater of the submarine mud volcano (Fig. 3, Gas). We interpret this plume as representing a continuous supply of new gas bubbles from the sea bed. This feature could also represent a shoal of ®sh. However, its conical plume shape, its position immediately above the submarine mud volcano and the occurrence of the assumed gas-charged and overpressured sediments below, all suggest that it represents a gas seepage. 3.3. Sea ¯oor erosion Sea ¯oor erosion is common in this area. Some sea ¯oor erosion sites are very steep and deep, resulting in gullies and canyons. However, the erosion over the
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J. Chow et al. / Marine Geology 176 (2001) 55±63
Fig. 2. Conventional seismic re¯ection pro®le 320-30 showing the Liuchieuyu mud volcano (MV) and Kaoping Canyon (KP). Structural and sedimentary features such as debris ¯ow (DF), channel cuts (C), erosional truncation (E) and shallow faults (F) near the mud volcano can be observed. The numbers 1±4 represent sedimentary sequence boundaries of Pleistocene.
Fig. 3. UNIBOOM 1 seismic pro®le. MV is Liuchieuyu submarine mud volcano. Gas seepage can be observed from the top left corner of the crater. F is a fault scarp and G a gully incision. The inset shows an enlargement of the small mud volcano on the left.
J. Chow et al. / Marine Geology 176 (2001) 55±63
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Fig. 4. Liuchieuyu Island and Kaoping Canyon. T1 and T2 mark the two bends in present-day Kaoping Canyon (thick line). MV is the mud volcano associated with bend T1. The thick dashed line indicates the route of the paleo-canyon. P indicates the area where traces of the paleocanyon still exist. The solid squares are mud diapirs and the stars represent mud volcanoes in Kaoping Canyon. Seismic image of the mud volcano A is shown in Fig. 7. FC is part of Fangliao Submarine Canyon. The inset shows the locations of shallow faults, small mud volcanoes and displayed seismic sections around Liuchieuyu mud volcano.
top of mud diapirs in UNIBOOM seismic section 3 (Fig. 5) appears to be associated with the uplifting of the mud diapir. The overlying sediments have been removed at some time with no apparent subsequent sedimentary drape over the angular erosional surface (Fig. 5). 3.4. Faults Many small shallow faults were found around the Liuchieuyu mud volcano. Some of these faults appear
on the high-resolution UNIBOOM seismic pro®les as an abrupt discontinuity in sedimentary bed re¯ectors that are otherwise well correlated on both sides of the fault (e.g. Fig. 5F), but fresh fault scarps can also be seen on the present sea ¯oor (e.g. Fig. 3F). None of these faults were extended, and they were mostly located around the Liuchieuyu mud volcano (Fig. 4, inset). Following Berryhill (1987), we hypothesize that these small faults were caused by the differential loading forces that accompany mud diapirism. When a mud diapir and/or mud volcano rises, the
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Fig. 5. UNIBOOM 3 seismic pro®le. Erosion and channel incision are common near the mud volcano. F are faults and G is a gully incision.
surrounding strata are also in¯uenced by a rising force. The nearer the strata are to the diapir or mud volcano, the stronger the rising force. However, as the mud diapirs are drawn from the source beds, the beds above the newly emptied space will subside, and where the rising and subsidence forces occur adjacently, faults will occur. 3.5. Small mud volcanoes The UNIBOOM sections also show several small mud volcanoes in the area to the northeast of Liuchieuyu mud volcano (e.g. Fig. 3). These conical structures are less than 200 m wide and about 4 m high above the sea¯oor, and they do not appear to be aligned with any of the faults (Fig. 4, inset). The surrounding strata are tilted and distorted. The acoustic voids beneath these small mud volcanoes appear to almost pierce the sedimentary strata, but it is also possible that these voids are arti®cial phenomena since the conical geometry alone might have caused a loss of seismic energy. Nevertheless, these sea¯oor features clearly are small mud volcanoes, and
as such they are also presumably connected to the large underground part of Liuchieuyu mud volcano.
There are two distinct bends near the head of the Kaoping Submarine Canyon (Fig. 4, T1 and T2), and they are both associated with submarine mud volcanoes (i.e. MV in Fig. 4 and Liuchieuyu mud volcano). We hypothesize that the eruption of these respective mud volcanoes in the original channel of Kaoping Canyon led to the formation of both the dogleg bend near the estuary of the Kaopinghsi River (Fig. 4, T1), and the other, sharper bend towards Liuchieuyu Island (Fig. 4, T2). Similar diversions in submarine canyons have been reported elsewhere. For instance, in Fangliao Submarine Canyon, which is about 25 km to the southeast of the study site (see Fig. 1 inset, FC), an uplifted mud volcano has caused a bend in the canyon (Yu and Lu, 1995). The Var Canyon in the Liguro±Provencal Basin also has a sharp bend when the canyon runs into evaporitic
J. Chow et al. / Marine Geology 176 (2001) 55±63
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Fig. 6. Graphic representation of the origin of the Liuchieuyu mud volcano. (1) The overlying sediment load prevented unrestrained growth of the original mud diapir. (2) Gas was trapped by the overlying sediment load to form a high-pressure zone. (3) Kaoping paleo-canyon eroded the overlying sediment away, the con®ning load became insuf®cient, and the mud diapir, augmented by the high-pressure zone, grew rapidly upward. (4) The mud diapir ®nally made its way to the sea ¯oor and became a submarine mud volcano.
massifs (Pautot et al., 1984). In the case of Kaoping Canyon, we further hypothesize that before it was diverted at T2, the paleo-canyon originally used to cut through the present Liuchieuyu area (see Fig. 4 for the projected channel of the original paleocanyon). In support of this, we interpret the seismic features labelled `C' in Fig. 2 as paleo-channel cuts incised by the original canyon. In addition, although the bathymetry of the area to the southwest of Liuchieuyu Island (Fig. 4, P) could be interpreted either as a rotational slump scar or as the ancient route of Kaoping Canyon, more evidence for the latter interpretation is provided by Wen (1991), who found traces of a paleo-canyon in 3.5 kHz seismic pro®les acquired in this area.
Even though there are many large mud diapirs and several small mud volcanoes in this offshore area, only the Liuchieuyu mud volcano has emerged
above sea level as a large mud volcano. While various models have been proposed to account for the formation of mud diapirs and mud volcanoes in different parts of the world (see e.g. Ogawa and Kobayashi, 1993; Hedberg, 1974; Henry et al., 1996; Yu and Lu, 1995; Vernette et al., 1992; Bishop, 1978; Berner et al., 1972), it is hard to explain completely the origin of Liuchieuyu mud volcano in terms of any of the seven causes listed in Barber et al. (1986), i.e. (1) sedimentary loading resulting from rapid sedimentation; (2) tectonic loading due to overthrusting; (3) abnormally high pore pressures developed in undercompacted shale formations; (4) gas generated from organic matter within shale; (5) dehydration of expandable clays through mineralogic transformation; (6) density inversion; and (7) faulting and seismic activity. We therefore propose here that Liuchieuyu submarine mud volcano was formed as a result of erosion of the sedimentary layers above a mud diapir and that this erosion was effected by the Kaoping paleo-canyon, which, as we have already argued, originally cut
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Fig. 7. Conventional seismic pro®le 320-24 showing a mud volcano in Kaoping Submarine Canyon. Location of the mud volcano is shown in Fig. 4.
through the site of the present-day mud volcano. The stages of this process are illustrated in Fig. 6. Stage 1: At the initial intrusion stage, Liuchieuyu mud diapir, that is, the precursor of Liuchieuyu mud volcano, lay entirely beneath the sea ¯oor. Layers of overlying sediment functioned as cap rock for the diapir and constrained its free growth. Stage 2: During this intrusion stage, gas migrated upward from deeper strata, but could not escape into the sea because of the cap rock. The migrating gas then accumulated on top of the diapir and a high pressure zone was formed. (Gas still escapes from present-day Liuchieuyu mud volcano as shown by the gas seepage in Fig. 3). Stage 3: The paleo-canyon (Kaoping Canyon) incised or eroded the sedimentary strata above the diapir. Stage 4: As the overlying strata eroded away, its constraining effect on the upward growth of the diapir
would progressively be reduced, until ®nally a large amount of high-pressure gas would suddenly explode upward. Since this gas would be mixed with the mud in the mud diapir, the erupting mud diapir would thus become a mud volcano. After the eruption of Liuchieuyu mud volcano from the bottom of the canyon, its huge mass would be suf®cient to divert the paleo-canyon from its original course. Subsequently in the middle-Pleistocene, when the growing mud volcano rose near enough to sea level, a coral reef formed on its summit. This later became the present Liuchieuyu reef limestone. In addition to the Liuchieuyu Submarine mud volcano, Mud volcanoes were also found in Kaoping Canyon (Fig. 7) and Fangliao Canyon (Yu and Lu, 1995). Although none of these other mud volcanoes were as large as Liuchieuyu, this repetitive association
J. Chow et al. / Marine Geology 176 (2001) 55±63
of mud volcanoes with submarine canyons supports our proposal that some shallow marine submarine mud volcanoes are formed when the overburdening pressure is released by canyon erosion. This model, which postulates a relationship between submarine canyons and the origin of mud volcanoes might also be applied to other mud volcanoes in various parts of the world.
From our analysis of seismic sections in the vicinity of Liuchieuyu Island, we draw two main conclusions. First, the two distinctive bends near the head of Kaoping Submarine Canyon were caused by mud volcanoes that erupted in the path of the canyon and blocked the channel. These detours in Kaoping Canyon are similar to those found in nearby Fangliao Submarine Canyon and also in the Var Canyon in the Liguro±Provencal Basin. Second, we propose that Liuchieuyu mud volcano was formed from a mud diapir by the incision of Kaoping paleo-canyon. This model postulates that initially the overlying sediment load prevented uncon®ned growth of the Liuchieuyu mud diapir and simultaneously acted as a seal that trapped gas in a highpressure zone. Subsequently, Kaoping Canyon eroded the overlying sediment away until there was no longer suf®cient sediment to constrain the diapir. The mud diapir, augmented by the high-pressure zone, then grew upward through the sea ¯oor and became Liuchieuyu mud volcano. This model is probably applicable to the formation of other submarine mud volcanoes adjacent to submarine canyons in other areas.
This research was supported by the National Science Council of the Republic of China. We express our appreciation to the of®cers and crew of the R/V Ocean Researcher I for providing support for seismic survey.
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Barber, A.J., Tjokrosapoetro, S., Charlton, T.R., 1986. Mud volcanos, shale diapirs, wrench faults, and melanges in accretionary complexes, eastern Indonesia. AAPG Bull. 70, 1729±1741. Berner, H., Ramberg, H., Stephansson, O., 1972. Diapirism in theory and experiment. Tectonophysics 15, 197±218. Berryhill, H.L., 1987. Diapirism and faulting, continental shelf and upper continental slope off southwestern Louisiana. AAPG Stud. Geol. 23, 191±224. Bishop, R.S., 1978. Mechanism for emplacement of piercement diapirs. AAPG Bull. 62, 1561±1583. Bouma, A.H., Stelting, C.E., Feeley, M.H., 1983. High resolution seismic re¯ection pro®les. Seismic Expression of Structural Styles, AAPG Stud. Geo1. 15, 1±23. Bouma, A.H., Stelting, C.E., Feeley, M.H., 1989. High resolution seismic re¯ection pro®les. Atlas of Seismic Stratigraphy, Bally, A.W. (Ed.). AAPG Stud. Geol. 27, 72±74. Chow, J., Lai, T.D., Liu, C.S., 1996. Flower structures and strikeslip deformation off southwestern Taiwan. Terrestrial, Atmos. Oceanic Sci. 17, 523±533. Covey, M., 1984. Lithofacies analysis and basin reconstruction, Plio-Pleistocene western Taiwan foredeep. Petro. Geol. Taiwan 20, 53±83. Hedberg, H.D., 1974. Relation of methane generation to undercompacted shales, shale diapirs, and mud volcanoes. AAPG Bull. 58, 661±673. Henry, P., Le Pichon, X., Lallemant, S., Lance, S., Martin, J.B., Foucher, J.P., Fiala-Medioni, A., Rostek, F., Guilhaumou, N., Pranal, V., Castrec, M., 1996. Fluid ¯ow in and around a mud volcano ®eld seaward of the Barbados accretionary wedge: results from Manon cruise. J. Geophys. Res. 101, 20297±20323. Ogawa, Y., Kobayashi, K., 1993. Mud ridge on the crest of the outer swell off Japan Trench. Mar. Geol. 111, 1±6. Pautot, G., Le Cann, C., Coutelle, A., Mart, Y., 1984. Morphology and extension of the evaporitic structures of the LiguroProvencËal Basin, new Sea-Beam data. Mar. Geol. 55, 387±409. Prior, D.B., Doyle, E.H., Kaluza, M.J., 1989. Evidence for sediment eruption on deep sea ¯oor, Gulf of Mexico. Science 243, 517± 519. Sangree, J.B., Widmier, J.M., 1979. Interpretation of depositional facies from seismic data. Geophysics 44, 131±160. Shih, T.T., Chang, J.C., Hsu, M.Y., 1991. The Marine Terraces Coral-Reef dating in Liu-Chiu Yu, Taiwan (in Chinese). Geography Research Report, National Taiwan Normal University 17, 85±97. Vernette, G., Mauffret, A., Bobier, C., Briceno, L., Gayet, J., 1992. Mud diapirism, fan sedimentation and strike-slip faulting, Caribbean Colombian Margin. Tectonophysics 202, 335±349. Wen, K.Y., 1991. 3.5KHz echo sounding and topography of Kaoping continental shelf, southwestern offshore area (in Chinese). Master thesis, National Taiwan University, pp. 155. Yu, H.S., Lu, J.C., 1995. Development of the shale diapir-controlled Fangliao Canyon on the continental slope off southwestern Taiwan. J. Southeast Asian Earth Sci. 11, 265±276.
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A submarine canyon as the cause of a mud volcano Ð Liuchieuyu Island in Taiwan J. Chow a,*, J.S. Lee a, C.S. Liu b, B.D. Lee c, J.S. Watkins d a
Institute of Applied Geophysics, National Taiwan Ocean University, 2, Peiling Road, Keelung, 20224 Taiwan, ROC b Institute of Oceanography, National Taiwan University, Taiwan, People's Republic of China c Energy and Resources Laboratory, Industrial Technology Research Institute, People's Republic of China d Department of Geology and Geophysics, Texas A & M University, College Station, TX 77843-3115, USA Received 21 July 2000; accepted 21 March 2001
In this paper, based on 3.5 kHz, UNIBOOM and conventional seismic data, we propose a model for the creation of Liuchieuyu Island, a near-shore mud volcano off the southwestern coast of Taiwan. In support of this model, we also discuss the relationship between a nearby submarine canyon (Kaoping Submarine Canyon) and the mud diapirs and mud volcanoes in the region. Seismic data suggest that Liuchieuyu was originally a mud diapir with a thick, continuous accumulation of sediments on its upper surface. This sediment load or overburden prevented uncon®ned growth of the underlying mud diapir, while simultaneously acting as a seal that trapped gas and formed a high-pressure zone. However, Kaoping paleo-canyon eroded these overlying sediments, and the overburdening pressure was reduced. In consequence, the mud diapir rose up to the sea ¯oor to become a mud volcano, the uppermost part of which is present-day Liuchieuyu Island. The formation of this island in the path of the paleo-canyon also diverted the channel of the canyon to its present location. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Taiwan; Seismic re¯ection; Submarine canyon; Mud volcano
In the early Pliocene, the paleoenvironment of the offshore area of southwestern Taiwan was deep marine (Covey, 1984). Large amounts of mud and silt derived from the Penglai orogeny were deposited in this region during that time, and in the Pliocene, a thick ®ne-grain sediment sequence was formed. However, as the arc-continent collision developed, the depositional environment in this area became
* Corresponding author. Tel.: 1886-2-24622192 ext. 6314; fax: 1886-2-24625038. E-mail address: [email protected] (J. Chow).
shallow marine and deltaic, and the compressive tectonic forces of this still-active collision together with the unbalanced sediment load combined to trigger the mud diapirism in this region. Disturbance of the very young sedimentary strata on top of several mud diapirs suggests that they are still active (Chow et al., 1996). Liuchieuyu Island, which is about 12 km off the southwestern coast of Taiwan (Fig. 1), is the only mud volcano in this offshore area to have emerged above sea level, even though there are many mud diapirs nearby. The island is located opposite the mouth of the Kaopinghsi River, and next to the Kaoping Submarine Canyon. Its northeast±southwest axis is about 5 km long, and its maximum width in the
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00157-8
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J. Chow et al. / Marine Geology 176 (2001) 55±63
Fig. 1. Map showing the locations of Liuchieuyu Island, the UNIBOOM seismic pro®les (U1±U4), the 3.5 kHz pro®les (A±J) and the conventional seismic sections (inset). Bathymetric contour is at 50 m interval. FC in the inset is Fangliao Canyon. The thick lines on the UNIBOOM traces and on the conventional seismics in the inset indicate the seismic sections shown in the corresponding ®gures.
northwest±southeast direction is 3 km. The total area above sea level is about 10 square km. The island is composed of greenish-gray mudstone from the late Pliocene, covered with a 4±10 m thickness of reef limestone that was deposited in the Pleistocene (Shih et al., 1991).
2.1. Conventional re¯ection seismic sections Two-channel re¯ection seismic pro®les were acquired aboard the R/V Ocean Researcher I during
J. Chow et al. / Marine Geology 176 (2001) 55±63
57
cruise 320 (Fig. 1, inset). The total length of these pro®les was about 521 km. The seismic source used was an array of three Bolt airguns with a maximum output volume of 920 in 3. (500 in 3 plus 300 in 3. plus 120 in 3.) at a pressure of 2000 PSI. Positioning was maintained using a Trimble Nav Graphic XL Global Positioning System and all the seismic data were digitally recorded. The seismic re¯ection data were processed using the SIOSEIS seismic data processing system.
was good enough not to require any subsequent special processing of the digital record.
2.2. High-resolution shallow seismic data
3.1. Liuchieuyu mud volcanoes
Since the high seismic source frequency of UNIBOOM pro®les makes them very suitable for high-resolution study of shallow shelf areas, four NW±SE UNIBOOM pro®les, of total length 34.8 km, were acquired northeast of Liuchieuyu Island (Fig. 1). These pro®les run from the margin of the Kaoping Submarine Canyon in the NW, to the coastal shelf in the SE. The seismograph used was the UNIBOOM 230-1 (EG&G) provided by Energy and Resources Laboratories, ITRI. This UNIBOOM system provides a sonic-impulse seismic source with an energy of up to 300 J. The generation rate is 3.3 impulses per second with a main frequency range of 400 Hz , 14 kHz. In 300 m of water, the stratigraphic resolution is 15 , 30 cm, and penetration depth is 30±60 m. During acquisition, the vessel speed was kept at about four knots and a Trimble Nav Graphic XL Global Positioning System was used to maintain position and record the pro®le location. Data was recorded up to a two-way time of 250 ms. Since the quality of the pro®les was good, no processing was done on the data set except for the bandpass ®ltering (low cut 800 Hz, high cut 4 kHz) used during the acquisition.
To the northeast of Liuchieuyu Island, the conventional seismic section (Fig. 2) and the high-resolution UNIBOOM shallow seismic pro®les (see Fig. 3) show a structureless mud volcano, about one km wide. This submarine mud volcano pierces the sedimentary cover, and the surrounding sedimentary strata dip away from it (Fig. 2 and 3). Fig. 3 also shows a crater that was formed by the natural rapid release of large volumes of over-pressured gas. Structurally, this crater conforms to the three-dimensional model used by Prior et al. (1989) to interpret a similar mud volcano crater in the Gulf of Mexico. These data further con®rm that Liuchieuyu Island itself is the above-sea-level part of a large submarine mud volcano. The extent of the submarine part of this volcano is indicated in Fig. 4.
2.3. 3.5 kHz Echo pro®les During cruise 434 of the R/V Ocean Researcher I, about 130 km of 3.5 kHz echo pro®les were acquired. These pro®les were located around Liuchieuyu Island, with some of them crossing Kaoping Submarine Canyon (Fig. 1). During the acquisition of the 3.5 kHz pro®les, the vessel speed was kept about at 5.5 , 6.5 knots, and the on-board GPS was used to record the pro®le location. The quality of the data
The following interpretations of the seismic pro®les are based on analytical techniques that have been described elsewhere (e.g. Sangree and Widmier, 1979; Bouma et al., 1983, 1989). Fig. 1 shows the locations of all the seismics discussed below.
3.2. Gas seepage The presence of gas is suggested by a small, acoustically re¯ective plume in the water column immediately above the crater of the submarine mud volcano (Fig. 3, Gas). We interpret this plume as representing a continuous supply of new gas bubbles from the sea bed. This feature could also represent a shoal of ®sh. However, its conical plume shape, its position immediately above the submarine mud volcano and the occurrence of the assumed gas-charged and overpressured sediments below, all suggest that it represents a gas seepage. 3.3. Sea ¯oor erosion Sea ¯oor erosion is common in this area. Some sea ¯oor erosion sites are very steep and deep, resulting in gullies and canyons. However, the erosion over the
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J. Chow et al. / Marine Geology 176 (2001) 55±63
Fig. 2. Conventional seismic re¯ection pro®le 320-30 showing the Liuchieuyu mud volcano (MV) and Kaoping Canyon (KP). Structural and sedimentary features such as debris ¯ow (DF), channel cuts (C), erosional truncation (E) and shallow faults (F) near the mud volcano can be observed. The numbers 1±4 represent sedimentary sequence boundaries of Pleistocene.
Fig. 3. UNIBOOM 1 seismic pro®le. MV is Liuchieuyu submarine mud volcano. Gas seepage can be observed from the top left corner of the crater. F is a fault scarp and G a gully incision. The inset shows an enlargement of the small mud volcano on the left.
J. Chow et al. / Marine Geology 176 (2001) 55±63
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Fig. 4. Liuchieuyu Island and Kaoping Canyon. T1 and T2 mark the two bends in present-day Kaoping Canyon (thick line). MV is the mud volcano associated with bend T1. The thick dashed line indicates the route of the paleo-canyon. P indicates the area where traces of the paleocanyon still exist. The solid squares are mud diapirs and the stars represent mud volcanoes in Kaoping Canyon. Seismic image of the mud volcano A is shown in Fig. 7. FC is part of Fangliao Submarine Canyon. The inset shows the locations of shallow faults, small mud volcanoes and displayed seismic sections around Liuchieuyu mud volcano.
top of mud diapirs in UNIBOOM seismic section 3 (Fig. 5) appears to be associated with the uplifting of the mud diapir. The overlying sediments have been removed at some time with no apparent subsequent sedimentary drape over the angular erosional surface (Fig. 5). 3.4. Faults Many small shallow faults were found around the Liuchieuyu mud volcano. Some of these faults appear
on the high-resolution UNIBOOM seismic pro®les as an abrupt discontinuity in sedimentary bed re¯ectors that are otherwise well correlated on both sides of the fault (e.g. Fig. 5F), but fresh fault scarps can also be seen on the present sea ¯oor (e.g. Fig. 3F). None of these faults were extended, and they were mostly located around the Liuchieuyu mud volcano (Fig. 4, inset). Following Berryhill (1987), we hypothesize that these small faults were caused by the differential loading forces that accompany mud diapirism. When a mud diapir and/or mud volcano rises, the
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Fig. 5. UNIBOOM 3 seismic pro®le. Erosion and channel incision are common near the mud volcano. F are faults and G is a gully incision.
surrounding strata are also in¯uenced by a rising force. The nearer the strata are to the diapir or mud volcano, the stronger the rising force. However, as the mud diapirs are drawn from the source beds, the beds above the newly emptied space will subside, and where the rising and subsidence forces occur adjacently, faults will occur. 3.5. Small mud volcanoes The UNIBOOM sections also show several small mud volcanoes in the area to the northeast of Liuchieuyu mud volcano (e.g. Fig. 3). These conical structures are less than 200 m wide and about 4 m high above the sea¯oor, and they do not appear to be aligned with any of the faults (Fig. 4, inset). The surrounding strata are tilted and distorted. The acoustic voids beneath these small mud volcanoes appear to almost pierce the sedimentary strata, but it is also possible that these voids are arti®cial phenomena since the conical geometry alone might have caused a loss of seismic energy. Nevertheless, these sea¯oor features clearly are small mud volcanoes, and
as such they are also presumably connected to the large underground part of Liuchieuyu mud volcano.
There are two distinct bends near the head of the Kaoping Submarine Canyon (Fig. 4, T1 and T2), and they are both associated with submarine mud volcanoes (i.e. MV in Fig. 4 and Liuchieuyu mud volcano). We hypothesize that the eruption of these respective mud volcanoes in the original channel of Kaoping Canyon led to the formation of both the dogleg bend near the estuary of the Kaopinghsi River (Fig. 4, T1), and the other, sharper bend towards Liuchieuyu Island (Fig. 4, T2). Similar diversions in submarine canyons have been reported elsewhere. For instance, in Fangliao Submarine Canyon, which is about 25 km to the southeast of the study site (see Fig. 1 inset, FC), an uplifted mud volcano has caused a bend in the canyon (Yu and Lu, 1995). The Var Canyon in the Liguro±Provencal Basin also has a sharp bend when the canyon runs into evaporitic
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Fig. 6. Graphic representation of the origin of the Liuchieuyu mud volcano. (1) The overlying sediment load prevented unrestrained growth of the original mud diapir. (2) Gas was trapped by the overlying sediment load to form a high-pressure zone. (3) Kaoping paleo-canyon eroded the overlying sediment away, the con®ning load became insuf®cient, and the mud diapir, augmented by the high-pressure zone, grew rapidly upward. (4) The mud diapir ®nally made its way to the sea ¯oor and became a submarine mud volcano.
massifs (Pautot et al., 1984). In the case of Kaoping Canyon, we further hypothesize that before it was diverted at T2, the paleo-canyon originally used to cut through the present Liuchieuyu area (see Fig. 4 for the projected channel of the original paleocanyon). In support of this, we interpret the seismic features labelled `C' in Fig. 2 as paleo-channel cuts incised by the original canyon. In addition, although the bathymetry of the area to the southwest of Liuchieuyu Island (Fig. 4, P) could be interpreted either as a rotational slump scar or as the ancient route of Kaoping Canyon, more evidence for the latter interpretation is provided by Wen (1991), who found traces of a paleo-canyon in 3.5 kHz seismic pro®les acquired in this area.
Even though there are many large mud diapirs and several small mud volcanoes in this offshore area, only the Liuchieuyu mud volcano has emerged
above sea level as a large mud volcano. While various models have been proposed to account for the formation of mud diapirs and mud volcanoes in different parts of the world (see e.g. Ogawa and Kobayashi, 1993; Hedberg, 1974; Henry et al., 1996; Yu and Lu, 1995; Vernette et al., 1992; Bishop, 1978; Berner et al., 1972), it is hard to explain completely the origin of Liuchieuyu mud volcano in terms of any of the seven causes listed in Barber et al. (1986), i.e. (1) sedimentary loading resulting from rapid sedimentation; (2) tectonic loading due to overthrusting; (3) abnormally high pore pressures developed in undercompacted shale formations; (4) gas generated from organic matter within shale; (5) dehydration of expandable clays through mineralogic transformation; (6) density inversion; and (7) faulting and seismic activity. We therefore propose here that Liuchieuyu submarine mud volcano was formed as a result of erosion of the sedimentary layers above a mud diapir and that this erosion was effected by the Kaoping paleo-canyon, which, as we have already argued, originally cut
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Fig. 7. Conventional seismic pro®le 320-24 showing a mud volcano in Kaoping Submarine Canyon. Location of the mud volcano is shown in Fig. 4.
through the site of the present-day mud volcano. The stages of this process are illustrated in Fig. 6. Stage 1: At the initial intrusion stage, Liuchieuyu mud diapir, that is, the precursor of Liuchieuyu mud volcano, lay entirely beneath the sea ¯oor. Layers of overlying sediment functioned as cap rock for the diapir and constrained its free growth. Stage 2: During this intrusion stage, gas migrated upward from deeper strata, but could not escape into the sea because of the cap rock. The migrating gas then accumulated on top of the diapir and a high pressure zone was formed. (Gas still escapes from present-day Liuchieuyu mud volcano as shown by the gas seepage in Fig. 3). Stage 3: The paleo-canyon (Kaoping Canyon) incised or eroded the sedimentary strata above the diapir. Stage 4: As the overlying strata eroded away, its constraining effect on the upward growth of the diapir
would progressively be reduced, until ®nally a large amount of high-pressure gas would suddenly explode upward. Since this gas would be mixed with the mud in the mud diapir, the erupting mud diapir would thus become a mud volcano. After the eruption of Liuchieuyu mud volcano from the bottom of the canyon, its huge mass would be suf®cient to divert the paleo-canyon from its original course. Subsequently in the middle-Pleistocene, when the growing mud volcano rose near enough to sea level, a coral reef formed on its summit. This later became the present Liuchieuyu reef limestone. In addition to the Liuchieuyu Submarine mud volcano, Mud volcanoes were also found in Kaoping Canyon (Fig. 7) and Fangliao Canyon (Yu and Lu, 1995). Although none of these other mud volcanoes were as large as Liuchieuyu, this repetitive association
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of mud volcanoes with submarine canyons supports our proposal that some shallow marine submarine mud volcanoes are formed when the overburdening pressure is released by canyon erosion. This model, which postulates a relationship between submarine canyons and the origin of mud volcanoes might also be applied to other mud volcanoes in various parts of the world.
From our analysis of seismic sections in the vicinity of Liuchieuyu Island, we draw two main conclusions. First, the two distinctive bends near the head of Kaoping Submarine Canyon were caused by mud volcanoes that erupted in the path of the canyon and blocked the channel. These detours in Kaoping Canyon are similar to those found in nearby Fangliao Submarine Canyon and also in the Var Canyon in the Liguro±Provencal Basin. Second, we propose that Liuchieuyu mud volcano was formed from a mud diapir by the incision of Kaoping paleo-canyon. This model postulates that initially the overlying sediment load prevented uncon®ned growth of the Liuchieuyu mud diapir and simultaneously acted as a seal that trapped gas in a highpressure zone. Subsequently, Kaoping Canyon eroded the overlying sediment away until there was no longer suf®cient sediment to constrain the diapir. The mud diapir, augmented by the high-pressure zone, then grew upward through the sea ¯oor and became Liuchieuyu mud volcano. This model is probably applicable to the formation of other submarine mud volcanoes adjacent to submarine canyons in other areas.
This research was supported by the National Science Council of the Republic of China. We express our appreciation to the of®cers and crew of the R/V Ocean Researcher I for providing support for seismic survey.
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